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Effects of long-term application ofvarious fertilizers and crops on soil microbiomes an a long-term field experiment were investigated using the library of the 16S rRNA gene sequences obtained by high-throughput sequencing of the total DNA. The communities exhibited high diversity, with 655 microbial genera belonging to 34 phyla detected (31 bacterial and 3 archaeal ones). For analysis of the effect of the studied factors on community structure, a linear model was developed in order to simplify interpretation of the data of high-throughput sequencing and to obtain biologically important information. Liming was shown to modulate the effect of mineral fertilizers on the structure of microbial populations. The differences in the structure and alpha-diversity of microbial communities were shown to depend more on the crops and liming, rather than on the fertilizers applied. Interaction between the crop factor and liming expressed as an ambiguous effect of liming on the microbiome in the presence of different plants was reliably demonstrated. Thus, in the case of barley and clover, liming resulted in increased taxonomic diversity of the community, while in the case of potato and flax it had an opposite effect.

The processes of speciation and macroevolution of root nodule bacteria (rhizobia), based on deep rearrangements of their genomes and occurring in the N₂-fixing symbiotic system, are reconstructed. At the first stage of rhizobial evolution, transformation of free-living diazotrophs (related to Rhodopseudomonas) to symbiotic N₂-fixers (Bradyrhizobium) occurred due to the acquisition of the fix gene system, which is responsible for providing nitrogenase with electrons and reducing equivalents, as well as for oxygen-dependent regulation of nitrogenase synthesis in planta, and then of the nod genes responsible for the synthesis of the lipo- chito-oligosaccharide Nod factors, which induce root nodule development. The subsequent rearrangements of bacterial genomes included: (1) increased volume of hereditary information supported by species, genera (pan-genome), and individual strains; (2) transition from the unitary genome to a multicomponent one; and (3) enhanced levels of bacterial genetic plasticity and horizontal gene transfer, resulting in formation of new genera, of which Mesorhizobium, Rhizobium, and Sinorhizobium are the largest, and of over 100 species. Rhizobial evolution caused by development and diversification of the Nod factor synthesizing systems may result in both increased host specificity range (transition of Bradyrhizobium from autotrophic to symbiotrophic carbon metabolism in interaction with a broad spectrum of legumes) and to its contraction (transition of Rhizobium and Sinorhizobium to "altruistic" interaction with legumes of the galegoid clade). Reconstruction of the evolutionary pathway from symbiotic N₂-fixers to their free-living ancestors makes it possible to initiate the studies based on up-to-date genome screening technologies and aimed at the issues of genetic integration of organisms into supracpecies complexes, ratios of the macro- and microevolutionary mechanisms, and developmetn of cooperative adaptations based on altruistic relationship between the symbiotic partners.

Using the examples of diverse interactions among prokaryotes and eukaryotes, the relationships between molecular and population mechanisms of evolution of symbiotic bacteria are addressed. Their circulation in host-environment systems activates microevolutionary factors that direct combinative or reductive genome evolution in facultative, ecologically obligatory, and genetically obligatory symbioses. Due to intense systemic intra-genome rearrangements and horizontal gene transfer, two types of gene systems evolve in these bacteria: (1) controlling the pathogenesis-like processes of host recognition and penetration and (2) responsible for mutualistic interactions that are related to nitrogen fixation and its transfer to the host. The evolution of gene systems of type 1 is directed by individual (Darwinian, frequency-dependent) selection, which is responsible for gene-for-gene interactions between the partners. In the evolution of the type 2 systems, group (interdeme, kin) selection plays the key role, being responsible for the development of bacterial traits beneficial for the host. Using the legume--rhizobia symbiosis as an example, it is shown that evolution of mutualism can be described in terms of biological altruism, whose regularities are common for intraspecific and interspecific relationships. Macroevolutionary rearrangements of bacterial genomes result from the structural changes in their populations, wherein various selection modes are combined with stochastic processes (genetic drift, population waves) induced in the symbiotic systems.

The polytypic species of nodule bacteria Rhizobium leguminosarum (includes biovars viciae and trifolii) and Neorhizobium galegae (biovars orientalis and officinalis) differ in the nucleotide polymorphism of housekeeping genes (hkg) and of symbiotically specialized genes (sym) that control the formation of N2-fixing nodules in leguminous plants. In R. leguminosarum, p-distance values for sym genes are higher than for hkg genes in strains from the same and from different biovars. In N. galegae, differences between biovars in sym genes are higher than in hkg genes, while within biovars, polymorphism in sym genes is lower than in hkg genes. Coefficients of biovar differentiation for both groups of genes are higher in N. galegae than in R. leguminosarum, possibly reflecting the spatial isolation of N. galegae biovars. In these species of rhizobia, the phylogenetic congruence of sym and hkg genes is more pronounced in N. galegae than in R. leguminosarum. This difference indicates an active transfer of sym genes in R. leguminosarum populations possibly representing an important factor of the deep diversification for symbiotic traits in this rhizobia species. [ABSTRACT FROM AUTHOR]

The taxonomic structure of the microbiomes colonizing the rapeseed rhizosphere soil was investigated using high-throughput sequencing of the 16S rRNA gene amplicon libraries. Effect of introduction of bacteria with ACC deaminase activity into the rapeseed rhizosphere community on rapeseed yeild was studied. In the prokaryotic complex, Proteobacteria, Actinobacteria, Gemmatimonadetes, and Acidobacteria predominated. An inverse correlation was found between abundance of the taxa Crenarchaeota and Proteobacteria (r = ‒0.71, P < 0.01). All detected archaeal sequences belonged to the family Nitrososphaeraceae, which is directly involved in intensification of plant nitrogen nutrition. Independent of the mineral nutrition, addition of ACC-utilizing strain resulted in a significant decrease in the shares of Crenarchaeota (63‒72%), Verrucomicrobia (42‒53%), and Bacteroidetes (50‒56%) and an increase in the share of the Proteobacteria (38%). Introduced bacteria were shown to have a positive effect on the relative abundance of the metabolically significant Proteobacteria classes in the rapeseed rhizosphere due to improved condition of the plants. A reliable correlation was found between relative abundance of the Proteobacteria and production of the plant‒microbial system (r = 0.85, P < 0.01) on addition of the strains Pseudomonas oryzihabitans Ep4 and Variovorax paradoxus 3-P4, which possess an ACC deaminase complex. Observed differences in the composition of microbial populations were primarily associated with the effect of introduced strains, rather than with addition of mineral fertilizers. [ABSTRACT FROM AUTHOR]

Two hundred forty-three isolates of alfalfa root-nodule bacteria ( Sinorhizobium meliloti) were obtained from nodules and soils sampled in the northern Aral region, experiencing secondary salinization. Isolates obtained from nodules (N isolates) were significantly more salt-tolerant than those from soils (T isolates) when grown in a liquid medium with 3.5% NaCl. It was found that wild species of alfalfa, melilot, and trigonella preferably formed symbioses with salt-tolerant root-nodule bacteria in both salinized and nonsalinized soils. Only two alfalfa species, Medicago falcata and M. trautvetteri, formed efficient symbioses in soils contrasting in salinity. The formation of efficient symbiosis with alfalfa in the presence of 0.6% NaCl was studied in 36 isolates (N and T) differing in salt tolerance and symbiotic efficiency. Fifteen isolates formed efficient symbioses in the presence of salt. The increase in the dry weight of the plants was 25–68% higher than in the control group. The efficiency of symbiotic interaction under salinization conditions depended on the symbiotic efficiency of the isolates under standard conditions but did not correlate with the source of root-nodule bacteria (soil or nodule) or their salt tolerance. The results indicate that the strains of root-nodule bacteria forming efficient symbioses under salinization conditions can be found. [ABSTRACT FROM AUTHOR]